Evaluation & visualization of double differentials in Mathematica

N_F

Hi, I'm trying to figure out how to use: http://www.wolframalpha.com/. I would like to be able to evaluate this function with different initial values and visualize results in some way, if possible:

[tex]d^2F = k/r^2 * i1*dl1 \times (i2*dl2 \times R)[/tex]


Evaluate and visualize: k= 5, i1=i2= 1, r= 1, R(z)=1, dl1=dl2=?
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1. CODE: D[1] D[1] 5/1^2 * 1*{x,0,0} cross 1*{y,0,0} cross {0,-1,0}

This produces some result and it even draws some surface.



2. CODE: D[ 5/1^2 * 1*{x,0,0} cross 1*{y,0,0} cross {0,-1,0}, x,y]

I think this one gives correct result, but it does not draw anything.


EXECUTE HERE: http://www.wolframalpha.com/input/


My problem is that I have no idea what to expect, I don't know if that function produces a surface or volume, or maybe even two lines or whatever, I only know the numerical result is supposed to be either: {0,k,0} or {0,2*k,0}, which with the above initial setup equals to: {0,5,0) or {0,10,0}, but then the result can not actually be a surface but a single vector? -- If "F" has units (U), then what would be units of double differential of "F": (U/m), (U/m^2), (U*m) or (U*m^2)?